Systemic lupus erythematosus (SLE) is a complex, autoimmune disorder predominantly affecting young females. Despite being multigenic, the HLA complex in general and HLA-DR in particular remains the most dominant genetic risk factor for disease susceptibility. A striking feature is the strong association of autoantibody specificities with some HLA haplotypes. This proposal addresses the mechanisms for this close association. In this proposal we will test the hypotheses that in lupus patients, molecular mimicry with microbial peptides is responsible for the selective enrichment of T cells reactive with lupus- associated autoantigens. Depending on microbial exposure, the HLA dictates the nature of cross- reactive peptides it binds and thereby the autoantigen selection. This process leads to activation of self- reactive T cells, autoantibody production, epitope spreading and end organ damage. Using Ro60 as the candidate autoantigen and HLA-DR and -DQ transgenic mice, following specific aims are proposed: to seek evidence for our hypothesis 1) To identify the molecular mimics of T cell epitopes on Ro60. 2) To demonstrate that multiple exposures to peptide mimics of Ro60 T cell epitopes influences the Ro60 reactive T cell repertoire. 3) To determine the pathogenic potential of anti-Ro60 initiated autoimmune responses in lupus-prone NZM2328 mice transgenic for HLA-DR3 and -DQ2. The findings from this proposal will clearly demonstrate that T cell responses to lupus-associated antigens can initiate autoimmune responses in SLE. This will shift the current paradigm that SLE is predominantly a B cell mediated disease to a more rational model in which both T and B cells have their unique roles in disease manifestation. This will provide a theoretical framework from which rational therapeutic approach can be devised.